KH only had one Sora falling from the sky and no Twilight Thorn as shown in the screens.
There's a very real chance that this is KH1.
Or more like "KH1: Final Final Mix + Reverse Rebirth"
That dream we read about in the demo could have occurred prior to Sora's Dive (in other words, directly prior to the start of KH1 when Sora starts having the "weird thoughts."). And then with the Riku-centric portions, my guess is that we may see, as Nomura suggested way back, Riku's time in the Dark Realm a la Reverse Rebirth.
As for why they would do yet another remake of KH1, there's actually boat loads of potential content they could add in light of the new games.
For instance, show Xemnas giving the memories to Vexen to create Xion. Have some additional introspective scenes for Xemnas and Xehanort's Heartless
to hint at such things as why Xemnas cannot use keyblades, and who, MX or Terra, is in more control of either of them.
Throw in some nods to Ven, particularly in any dream sequences, nostalgia in the same way that Sora felt nostalgia staring at the pic of the castle in Deep Jungle, and especially when Sora releases the hearts he contains. Show the creation of Namine (and perhaps extend Roxas' to... elaborate more, if you catch my drift).
Didn't Nomura say recently that he hoped to "put the series into perspective" or something of that nature? This could be it. It could be a game for people who have played the other KH games, serving a sort of purpose to connect the pieces and, of course, hint at the future (
Like elaborating more on this "torment.).